1. Field of the Invention
The present invention relates to an image sensing apparatus and an image data correction method, and more particularly to a technique for correcting deterioration of image quality due to optical factors based on optical information.
2. Description of the Related Art
Along with the recent trend toward digital video cameras and digital cameras with an increased number of pixels, high quality images having higher resolution are becoming available. On the other hand, deterioration of images due to pixel size reduction, and deterioration of image quality that is principally caused by the optical system, such as reduction in the quantity of light in the periphery of an image circle, magnification chromatic aberrations and distortion aberrations, due to factors such as difficulty in designing small lenses, are becoming more noticeable.
Deterioration of image quality due to optical factors as described above is considered to theoretically have point symmetrical phenomenal characteristics that spread radially from the center of the optical axis. However, the actual images are affected by the characteristics of a lens that is constituted by a plurality of lens groups, the shape of optical members, such as a diaphragm mechanism, as well as the physical structure of sensors, and thus the deterioration of image quality does not always have point symmetrical phenomenal characteristics.
To address such deterioration of image quality having point asymmetrical phenomenal characteristics, conventionally, correction techniques using parameter information (hereinafter referred to as “optical parameters”) of the optical system, such as focal length, aperture and focus position, have been proposed by, for example, Japanese Patent Nos. 3824237 and 3912404, and Japanese Patent Laid-Open Nos. 2005-341033 and 2007-134903.
When focal length, aperture and focus position constantly vary during image sensing, particularly in the case of a video camera, deterioration of image quality due to optical factors fluctuates successively depending on changes in the optical parameters. In addition, it is also necessary to give consideration to the fact that the memory capacity required to store correction characteristics data for correcting the above-described point asymmetrical deterioration of image quality, and resources such as the processing time of correction characteristics data are limited and few in the case of image sensing using a camera or a video camera.
In order to cope with such a situation, when generating correction characteristics data, it is necessary to perform appropriate control operations according to the state of optical characteristics and the state of image sensing while taking into consideration the point asymmetry of image quality deterioration.
According to Japanese Patent No. 3824237, the mixing ratio of optical correction values is determined according to optical parameters, and shading correction is performed according to the pixel distance obtained by correcting the distance from a pixel of interest to the center of the screen with distance gain functions in the vertical and horizontal directions. However, this technique only similarly changes basic correction characteristics using the distance gain functions. Accordingly, for optical characteristics having complex point asymmetry, it is necessary to have complex distance gain functions, and appropriate correction effects cannot be obtained since this technique cannot cope with shape changes that are dissimilar. Furthermore, approximating correction characteristics for changes in optical parameters and image height position with a simple model does not actually achieve an appropriate correction process.
According to Japanese Patent No. 3912404, the relative positions of the center of the optical axis and the center of the image sensor caused by image stabilization process are detected, and the phase of shading correction information stored discretely in a radial direction from the center of the optical axis is switched. However, in this example, the discretely disposed correction information is used without being processed, and thus for optical characteristics having complex point asymmetry, there is no way but to simply increase the density of the correction information.
According to Japanese Patent Laid-Open No. 2005-341033, in order to cope with the asymmetry of shading caused by the sensor structure, correction information stored in the x axis and y axis directions is made asymmetric, and the correction coefficient at the position of a pixel of interest is calculated. However, in this example, the asymmetry of the entire screen is expressed by synthesizing line-symmetrical phenomenal characteristics so that they can be expressed by a specific function, and thus this technique cannot cope with optical characteristics that change in an angular direction with respect to the center of the optical axis.
According to Japanese Patent Laid-Open No. 2007-134903, in order to cope with the asymmetry of shading caused by a distortion in the optical system, correction information is converted using an actual measured shading amount, and the correction coefficient at the position of a pixel of interest is calculated. However, this example requires actually measuring point asymmetrical characteristics and storing the information. If actual measurement is performed, there is no point in having correction information in the first place. In addition, it is realistically impossible to perform the same process in the state of actual image sensing in which the optical parameters successively change.